1. Field of the Invention
The present invention relates to a process for the transfer of information and a suitable system therefor.
2. Description of Related Art
In many sectors of technology waves are used for the transfer of information. These may be electromagnetic or acoustic waves, for example, which are disseminated either in a special conductor or freely in a given transfer medium, and in this way pass from the transmitter or transmission unit to the receiver or reception unit. With analog information transfer, the values which are to be transferred are formed into a stepless continuous spectrum of physical states. This occurs typically in the form of an amplitude, frequency, and/or phase modulation of the carrier waves. This enables very large volumes of information to be transferred in a given interval of time. With digital information transfer, by contrast, there is a restriction to specific discrete states. With regard to the transfer rate, however, if electromagnetic waves are being used, there have still been no restrictions encountered in practice hitherto, since the frequencies of the carrier waves concerned are very high, and different digital states can be realised in extremely short spaces of time.
In some transfer media, however, such as water for example, information transfer by electromagnetic waves is only possible to a limited degree, since these have only a short range. Accordingly, in this situation the use of sound waves for the transfer of information is a possibility, which can often be propagated over substantially greater distances. These sound waves, however, are mechanical pressure waves, which, apart from the substantially lower frequency, which naturally has an effect on the transferable information rate, also differ in respect of general propagation. Their propagation speed, for example, depends very much on the particular ambient conditions.
The wide range of problems which can arise with acoustic information transfer, can be illustrated briefly by the example of the transfer of sound signals under water. With the propagation in space of the sound waves emanating from a transmitter, a part of the waves may be reflected from the water surface and/or from the bed of the body of water, depending on the depth, from various objects, particles in suspension, and even from layered inhomogeneities in the water, or bent by them. The various different components of sound waves will then arrive at the receiver with differing amplitude and phase relationship, depending on the length of run, angle relationships, and acoustic properties of the relevant limit surfaces or media. As a consequence of the interference, the actual signal at the reception point may be amplified, weakened, distorted, or even totally deleted, in an unforeseeable manner, or reception may also be distorted by what is referred to as reverberation.
To explain the problems in greater detail, the simple situation will first be considered in which only a very short signal of a specific frequency, referred to as a CWP (Continuous Wave Pulse) is transmitted. In this situation (so-called Multipath Propagation), a receiver can obtain not only an individual signal, but a whole group of temporally-displaced individual pulses of different strengths. This effect is referred to as xe2x80x9cchannel responsexe2x80x9d. While in this case it is still possible for the individual pulses to be distinguished on the receiver side, and, for example, the most suitable pulse to be selected as the xe2x80x9cactual signalxe2x80x9d (whereupon the other pulses can, as a consequence, be regarded as xe2x80x9cinterference signalsxe2x80x9d and treated accordingly), a separation of this nature in the transmission of a longer wave package cannot normally be effected any longer, since the receiver receives only a summary or composed signal, which may indeed still have the same frequency as the initial signal, but in which the actual signal and the interference signals, with their different amplitudes and phase positions, are overlaid in such a way that unforeseeable fluctuations in the amplitude and also in the phase location may arise. This undesirable effect, which renders the evaluation of the signal difficult or can even, under certain circumstances, make this impossible, is referred to as xe2x80x9cIntersymbol Interactionxe2x80x9d (ISI). If transmitter and receiver move relative to one another, an additional problem may arise in the form of frequency shifts as a result of Doppler effects.
This wealth of problems makes underwater communications very difficult, such as by means of ultrasonics between divers and/or underwater vehicles, as well as the remote control of underwater equipment. Hitherto, analog information transfer in particular has only been practicable to a very limited degree. It was and is, however, still frequently used for the transfer of speech, whereby use is made of the fact that human beings can identify known words and sense associations even in cases of reception subject to very heavy noise interference. By appropriate practice and agreement on a restricted vocabulary, the identification rate can be somewhat improved. This process is not suitable, however, for transferring, for example, computer data or other information by mechanical means. Accordingly, in the acoustic information transfer sector too, suitable digital processes are being sought.
Today""s technical digital systems, especially for underwater use, are based mostly on the sequential transfer of sound signals of consistent height, which are located in a more or less narrow frequency band.
A further development represents broad band procedures (see e.g. U.S. Pat. No. 5,124,955) using a plurality (100) of parallel frequency channels. For reducing the influences of multipath propagation, these procedures use a stepwise switching between the frequency channels. Certain channels are provided for submitting a binary 1, while other channels are provided for submitting a binary 0. Five channels carry the same information, wherein the power portions of the redundant channel groups are added in the receiver and compared for reducing fading effects. Accordingly, the natural redundancy caused by the multipath propagation is reduced by the introduction of an additional synthetic redundancy (10 frequency channels are used for each bit). This common procedure is relatively stable. However, it does not allow modulations with an increased graduation.
Irrespective of whether the transmission takes place in a narrow or broad frequency band, encoding by means of serial xe2x80x9cclicksxe2x80x9d only allows for a limited information transfer rate. With a shortening of the pulses, the band broadening increases. Furthermore, Doppler effects may be compensated in a restricted manner only.
Another common multichannel system (see WO 99/19058) uses the so-called Orthogonal Frequency Division Multiplexing (OFDM) also for channels with constant frequencies in combination with a Forward Error Correction (FEC). This is in particular provided for a reduction of errors caused by the superposition of multipath components. This procedure is described as allowing a Differential Quadrature Phase Shift Key (DQPSK) modulation with bit rates up to 3000 bps (OF 31 carriers and FNR =10 dB) and up to 9600 bps (with 100 carriers). Unmodulated pilot signals with constant frequencies are transmitted above and below the frequency band used for information transmission for compensating Doppler effects. The frequencies of the pilot signals are permanently monitored with two separate PLL""s which submit corrections to a Discrete Fourier Transformer (DFT) unit. This procedure represents a complicate method which requires a complex technical equipment. Furthermore, this procedure uses the transmission physics in a restricted manner only.
The prior art development of transmission techniques is directed on complex post-transmission processing with complicate equalizers, PLL and correction algorithms which are implemented with the DSP technique. A further improvement has been obtained with the so-called beam forming (see e.g. O. R. Hinton et al. in xe2x80x9cSignal Processing VII: Theories and Applicationsxe2x80x9d, eds. M. Holt et al., European Association for Signal Processing, 1994, pp. 1540-1543). For the beam forming technique, the receiver is provided with an array of receiver elements to be focussed to certain multipath arrivals. However, this technique is restricted to short distance transmissions only.
It is known from solar and radar techniques that pulses with linear frequency modulation (LFM pulses or angel-modulated pulses) with a continuous frequency change have some advantages in particular under high noise conditions. These advantages comprise improved energy distribution, recognition, SNR as well as a higher system gain. Attempts have been made to use this effect in underwater communication. There are known some procedures in which a series of pulses with linear frequency change are serially transmitted instead of pulses with a constant frequency. It is known to discriminate between increasing and decreasing LFM signals additionally to the detection of the presence or non-presence (binary 1 or binary 0) in the ON phases. These procedures allow a switching of the frequency change direction only but not modulations with higher graduations.
It is known from mobile telephone transmissions to start information signals with a preamble of LFM pulses. This preamble or header facilitates the synchronisation in multiuser operation. Furthermore, both the high frequency transmission and the optical information transmission use procedures with a generation of mono-frequency pulses (so-called CW pulses) which have an extremely short duration and correspondingly a broad band characteristic (frequency broadening). These pulses are timely dilated with dispersion filters (so-called SAW or Surface Acoustic Wave filters) and serially transmitted in a predetermined frequency band. The heads of the LFM signals are located in predetermined narrow time slots. The broadened signals may have a time overlap during transmission which however does not destroy the principle of serial transmission. The signals are compressed in the receiver. For the dilation and compression, the same SAW filters are used in reversed directions. Depending on the arrangement of these elements, increasing or decreasing signals can be generated or demodulated. The amplitudes can be varied correspondingly. However, the modulation potential of these elements is restricted at this point. The frequency increase is delimited for technical reasons. With a shortening of CW-pulses, the frequency broadening and the duration of LFM signals is increased. The maximum length of LFM pulses is fixed due to the definition of the length and material of SAW filters. Each pulse can have one predetermined state only. Accordingly, it can transmit one information unit only. SAW filters are not usable for the decoding of low frequency acoustical signals in UW communication. Furthermore, SAW filters have tuning problems in particular as a result of multipath influences and Doppler shifts.
Attempts have been made to improve the transmission of serial LFM pulses by pulse-wise switching the start frequency (multiplexing on parallel frequency channels) in order to reduce the problems of multipath propagation. In U.S. Pat. No. 6,047,023, a mobile receiver is described which is capable to process longer LFM carrier signals. Basically, this technique corresponds to the above submission of short LFM pulses in predetermined time slots. The carrier signals are generated with other components only and subsequently demodulated. All carrier frequencies have the same gradient. Accordingly, all tracks are parallel to each other. The time slots have to be defined such that the tracks have a partial time overlap while the frequency bands always have to be separated from each other. With this procedure, complex modulation techniques can be used for information coding in the high frequency range only.
A general problem of commonly used LFM carrier signals is the following. On the one hand, down-stream equalizers show an increasing complexity. On the other hand, an appropriate equalization function cannot be formed without a compensation of the multipath spectrum contained in the received signals. Accordingly, additional distortions result. In the prior art, the presence of multipath arrivals (multipath components) with different arrival times has been considered as a problem which has to be solved with signal processing techniques. Multipath arrivals have never been used as a technical effect. This was in particular a result of the fact that the commonly used LFM carrier signals have small frequency gradients and inflexible structures.
Another problem of LFM signals is given by strong Doppler shifts which occur in particular in acoustic UW communication. Up to now, there is no procedure available which uses the advantages of a continuous frequency change of the carrier signals for a seperation of multipath components (so-called channel responses) by the timely synchron provision of a plurality of signal components forming a common system in a given frequency band which components can be used for a complete doppler compensation.
An object of the present invention is to provide a process or a suitable system for the transfer of information which will allow for a high transfer rate over long range.
That object is, further, to provide a process or system for the transfer of data which is resistant to the causes of interference referred to heretofore, and is capable of adaptation to different transfer conditions.
In particular, that object is to provide a suitable system for signal processing which is capable, with a high degree of selectivity and the best possible exclusion of intersymbol interaction, of always isolating and analysing as far as possible, from a large number of channel responses, those signal components with the smallest transfer losses.
A further object is to provide a process or suitable system for signal processing, which in the same context will guarantee the most complete compensation possible for Doppler effects.
A still further object is, by attaining the best possible quality of signal processing, to create the preconditions for a substantial increase in the transfer rate and, if applicable, also in the range, even under complicated transfer conditions, such as, for example, in the case of communication with or between moving objects under water.
According to the invention, an information signal is generated which consists of at least two signal components, at least one reference component (BK) and at least one information component (I1; I2; . . . ; IN), so that several frequency channels or components are available. By the simultaneous use of these, more information units can be transmitted per time unit. In addition, discrete states are provided by both the reference frequency channel or the reference component, as well as the information frequency channel or the information component, which form a bit pattern.
To provide the bit pattern in the simplest case, the frequencies or tones of the information frequency channels can be switched on or off, whereby the presence or absence of the signal frequency components concerned is evaluated as binary information (ON/OFF), i.e. 1 or 0. In this way it is therefore possible to transfer a bit on each of these information channels. The signal components together produce a bit pattern, in which the information can be encoded in any desired manner.
While this simplest case relates to practically all the parameters of the information signal concerned, it is however possible, in the ON states, for different signal parameters to be varied in such a way that a distinction can also be made between other digital states.
According to the invention, the frequency of at least one these components is timely continuously changed during the transmission. According to this measure, which is called Frequency Gradient Method (FGM) in the following, the influences of reflections and distortions on the transmission path can be eliminated.
After receiving the information signal, the at least one frequency variable component is transferred into constant intermediate frequencies. In the course of signal processing, the following feature is used for a separation of the signal components. Depending on the increase of the frequency gradients used in the transmitted signal, the run time differences of the multipath components contained in the received signal are represented in the form of frequency differences after the transfer into constant intermediate frequencies. The best signal components are selected from the spectra of constant intermediate frequencies (Zxe2x80x21; Zxe2x80x22; . . . ; Zxe2x80x2N+X), preferably by the use of filter devices. Subsequently, the relevant information parameters are evaluated.
A system for the transfer of information, being adapted to carry out a process according to the invention, comprises at least one transmitter unit and at least one receiver unit, between which an information signal (IS) is transmitted, wherein
the transmitter unit has a device for creating reference component (BK) and at least one information component (I1; I2; . . . ; IN), in order to generate temporally continuous frequency changes and to provide a bit pattern, and
the receiver unit contains a device for the acquisition of the information signal (IS) consisting of at least one information component (I1; I2; . . . ; IN) and one reference component (BK), in which at least one component has a temporally continuous frequency change.